To achieve reverse thrust in a boat it is normal to use a deflecting bucket, where the flow at the outlet is redirected backwards under the transom of the craft, through an angle normally greater than about 120 degrees to the direction of the outlet flow. This method generates sufficient thrust to make the craft go backwards, however the maximum reversing thrust is only about 40% of forward thrust. Additionally there are several unwanted drawbacks associated with reversing systems of this type.                1. The change in direction of the flow through the bucket generates an upward force as well as a reverse force. This upward force tends to lift the craft at the transom (stern) and apply a downwards force at the front (bow). In situations where the craft is used in shallow water operation, particularly to reverse off sandbars, reefs, shallows or the shore-line, the downwards force at the bow acts as an impediment to the reversing force. If the bow of the craft is lodged on the bottom or it is resting on the beach, the downwards force at the bow can cause the craft to founder and render the reverse ineffective.        2. The redirected water flow from the jet is at a downwards angle to avoid impacting the transom of the craft. If the reverse flow impacts the craft's transom or trimming structures mounted there, much of the reverse thrust is negated by the current associated with the flow.        3. The redirected water from the reverse bucket has a great deal of kinetic energy as well as a downwards component, which in shallow waters causes the bottom to be stirred up. In fragile and environmentally sensitive environments this high energy stream of water can cause unacceptable damage. Coral reefs, underwater weed and grass beds, shell fish, the end of launching ramps and shallow harbours etc can all be damaged by this high energy plume being re-directed downwards.        4. Once the bottom has been stirred-up, abrasive materials such as sand and in some cases coral become water-borne. All current commercial high-pressure water-jet pumps require tight tolerances between the pump housing walls and the impeller blade tips. Any ingested abrasive material will cause expensive damage to these pumps as the rotating components grind away the water lubricated bearings, pump housings and the impellers, resulting in loss of operational tolerances.        5. Additionally the reversing action in shallow waters can cause major damage to the internal components and structures inside conventional jets, as it is not only fine sands and material that can be ingested, but also larger bodies such as small stones. These stones and hard objects impact on the leading edges of the impellers and straightening vanes causing them to dull or blunt, resulting in loss of performance and economy.        6. If weed or general flotsam is disturbed by the action of the reverse bucket, this can be sucked onto the protective grill (a structure designed to prevent larger objects from entering the unit). Partial or total blockage of the grill can cause serious loss of propulsion.        
Reversing the flow through a pressure jet system is well known in the industry, however the purpose is to back-flush the grill bars when they become blocked. There are various reasons pressure jets benefit very little from reversing the flow:—                1. Fundamental to all commercial pressure jet systems is high pressure head in the pump section and this requires that the outlet, relative to the impeller diameter, is a reducing ratio. When the flow is reversed there is no mechanism for creating a nozzle and thus pressure head. The inlet side of a pressure jet is always larger than the outlet. The smallest part of an intake duct is where the duct merges with the impeller. Comparing the area of the outlet to the area of the impeller, the ratio is usually between about 1:3 & 1:4 for axial pumps and even greater for mixed flow pumps. When the flow is reversed the intake to outlet ratios preclude any significant pressure head from being produced. Instead, significant suction pressure is induced at the nozzle which causes serious cavitation on the impeller blades when higher rpm is applied.        2. If somehow the nozzle was made larger or removed and the flow is reversed, it is important that the flow has substantial mass. The designs of pressure pump impellers are counter-functional for the purpose, i.e. they have fine pitched blade angles (usually between 11-19 degrees of pitch) which are designed to generate pressure head and not pump large mass. Consequently they move significantly less water for each revolution of the impeller compared to a propeller. Effective and efficient thrust requires high mass of water to be passed through the impellers particularly at low craft speed The impellers have to be rotated at least double to three times the speed of a normal propeller of equivalent diameter to achieve the same mass through-put. The high speeds of the impellers at relatively low boat speed causes high risk of severe cavitation damage.        3. All commercial pressure jet systems utilize a pressure inducing impeller followed by a set of fixed straightening vanes. The purpose of these vanes is to remove the radial energy component added to the water by the rotating blades. Axial flow is necessary once the water is ejected into the atmosphere; otherwise the plume dissipates in a perpendicular direction to the desired flow, producing little thrust. Fixed position straightening vanes are always found downstream from the rotating impellers, however by reversing the flow the straightening vanes would now be positioned upstream from the impellers, thus acting as additional impedance to the in-flowing water.        4. An additional problem for reversing the direction of the water-flow through a pressure jets is the proximity of the nozzle or outlet to the water surface. It is considerably easier for air to be sucked into the system (ventilation) than it is for water to be drawn backwards through the nozzle. Because jets are designed to expel water above the water-line, even when the boat is off the plane, the nozzle section is near the water surface or even partially out of the water. Suction at the nozzle as a result of reversing the drive, can lead to severe ventilation (air drawn into the system) and as a consequence, loss of propulsion.        